The Foreign Office said last night it was monitoring the cases of three tourists from the French Indian Ocean island of La Réunion who apparently contracted the lethal H5N1 strain of bird flu while on holiday in Thailand.

The French authorities were waiting for further test results on the tourists who returned to La Réunion last week after spending seven days in Thailand. The travellers took pictures of each other at a bird park where they came into close contact with exotic species, according to Hélène Monard, a French health ministry spokeswoman.

Half a million Britons travel to Thailand on holiday each year and last night the Foreign Office said it was watching developments. "We are monitoring this closely," said a spokesman. "It is not 100% confirmed yet and we will consider what might need to be done in terms of updating advice to travellers once it is confirmed or not."

Britons travelling to Thailand and other areas where avian flu is present are advised against visiting markets where live poultry may be on sale. They are also warned to avoid contact with surfaces that may be contaminated with animal faeces and not to eat or handle undercooked or raw poultry, egg or duck dishes.

Meanwhile, Margaret Beckett, the environment secretary, has announced further emergency measures to the House of Commons to control the spread of avian flu to Britain. She said the government was seriously considering ordering free range poultry farmers to lock their chickens indoors to avoid infection from wild migrating birds. It is unclear whether farmers who are forced to shut their hens and chickens indoors will be able to keep their free range status.

Ms Beckett said bird fairs, exhibitions and markets would be banned in the next few days, and she urged anyone holding a fair in the days before the ban to postpone or cancel it. Across the UK the organisers of several bird fairs, from large speciality events to 10p-an-entry budgie exhibitions, were heeding her advice. The owners of 1,584 prize pigeons and poultry entered for the Countryside Live food and farming fair in Harrogate, North Yorkshire were being contacted yesterday. Another 3,000 exhibitors were given refunds by the East Midlands Bird Breeders' Association, which has called off its annual gathering in Newark, Nottinghamshire.

Amid confusion over the advice about eating chicken and raw eggs, Ms Beckett reiterated a warning that people should take extra care when eating poultry and make sure chicken was cooked properly. She advised against eating raw eggs.

But Peter Bradnock of the British Poultry Council (BPC) said the advice was no different to that which had been issued for years to avoid salmonella. He said there was no evidence that avian flu could be passed on through eating poultry meat. Judith Hilton, the head of microbiological safety at the Food Standards Agency, endorsed his comments, saying there was no evidence to suggest that transmission through digestion was a danger.

According to the BPC, British consumer confidence in poultry remains high. After a slight "wobble" in the market last weekend, the situation had stabilised and British consumers showed no signs of following the public in Italy, where chicken consumption has fallen by 40%.

In the La Réunion cases, a 43-year-old man was admitted to hospital on Saturday, three days after returning from Thailand with 19 others. He was suffering from flu-like symptoms, severe headaches and a hacking cough. One test on the man showed positive for H5N1 but a second proved inconclusive, according to the authorities on the island. Two fellow travellers also had flu symptoms and Ms Monard said their preliminary tests were also positive for H5N1.